"I left LA in the height of summer and I felt I was actually going to kill someone it was that horrible weather-wise. I went to Ireland and it was so beautiful, and for two years, I just didn't leave."
She has since moved back to LA, although she tries to divide her time between the two places. "I can work in LA or in Ireland. The theme of what I'm working on will stay the same wherever I am, but I am married and my husband works in LA, so it's a little bit difficult for him because he has an actual job," she laughs. Mercedes is married to musician Chris Watson, who plays guitar with Juliette Lewis among others.
The 38-year-old is the daughter of Gottfried Helnwein - a remarkable, often controversial force within the art world - and is now an established artist in her own right.
Gottfried's photo-realistic, larger-than-life paintings of children, often placed in tortuous or menacing situations, as well as his confrontation of Austria's Nazi past, have meant that he has been talked about, adored, reviled and much-referenced since his first show in Vienna in the mid-1960s.
Mercedes has a lighter, more illustrative touch, but the confrontation is there too. The women (they are mostly women) in her artworks are challenging and complex. They make eye contact through masks, or fringes, or cigarette smoke; aware but oblique. They all have secrets - and attitude.
Mercedes's career is clearly flourishing in LA - she has had regular solo shows since 2005, attended by critics and collectors, as well as a hefty smattering of celebrities including musicians Jack White, Beck and Juliette Lewis; her work is collected by everyone from "students with very limited funds", to "billionaires" and she has collaborated several times with Irish designer Orla Kiely. But the climate is less of a fit.
"My body prefers Irish weather," she says with a laugh. "It's really, really hard in LA - sun lotion, hats, never sitting out. Even driving in the car, the sun can damage your skin, so I wear sweaters in the car. It's crazy."
With her flame-coloured hair and alabaster complexion, Mercedes is a perfect Hollywood version of 'Irish', but in fact, she only moved to Ireland when she was in her teens, after a childhood spent between Germany and Austria, with occasional stints in America.
The family moved over when Mercedes, the second oldest of four, was aged 17. Brought up bilingual, she attended an American high school in Germany because "my dad knew how hard it was to get rid of a German accent".
Ireland was a random move in that there was no ostensible reason for it, but "we were all kind of intrigued by the country, I think", she says. "We didn't have any friends in Ireland, but we kind of fell in love with the country before we'd ever been there."
They arrived, late one Christmas Eve. "It was a very turbulent landing," she recalls. "We were in a storm, but we were so excited. The airport was empty, the streets were empty, because it was Christmas. My mum had found an apartment in Dublin, a base from which we could look for somewhere to live. We drove around and looked at a few places, one of them was the castle. It looked very different at the time. The gardens were bare, nothing planted, it was a bit Wuthering Heights. We just really liked it."
And so, in 1997, the Helnweins - dubbed by The New York Times 'The Real-Life Addams Family' - moved into Gurteen Castle in Tipperary. Did Mercedes mind living in the middle of nowhere as a teenager? "No," she says. "I was writing a lot even at that time, melodramatic stories and novels. I was always good at being a loner. My friends were all over the world - I wasn't used to seeing them that much anyway."
She recalls winter days where "the sun goes down at 4.30 in the afternoon", with affection, and seems to have embraced the isolation. "The thing about moving at 17 was that, if you don't go to school, you don't make a circle of friends, so it was lonely in that sense, but our family is pretty big, and I'm built in a way that I found that inspiring. It was so romantic. I was reading a lot of Victorian literature, and it all played into the fantasy I had at that period of my life."
The Helnweins have always seemed a self-sufficient family, a "gypsy tribe", as Gottfried has described it, happy in each other's company.
"Our parents raised us in a way that there was a closeness, a friendship," says Mercedes. "They've always been very supportive, people we could talk to and have fun with. I get along with my brothers really well. I know it's probably not the norm, but..." she laughs, "but we could just hang out, play trivia games, and be entertained".
Gottfried and his wife, Renate, do not seem to have had much in the way of teenage rebellion to deal with from herself or her three brothers, I suggest to Mercedes. She laughs. "The urge was never there," she agrees.
"If I had lived in a family that was a little bit more normal, where there were restrictions... in that case, I would have rebelled and done all the normal stuff - shaving my head, doing whatever necessary to shock. The thing about my parents that probably makes them different is they are not very small-minded. I think if I had grown up in a more conventional family... that's always the world I'm trying to get out of. I don't feel comfortable in it."
Displayed throughout the castle are huge, haunting works by Gottfried, giant canvases painted with children who might be sleeping or dead. Did Mercedes ever model for him? "Occasionally, but my brother, Ali, was more patient and easier as a model, and therefore used more often."
And yes, she knows that not everyone would enjoy living with the work. "We grew up with it. If anything seemed weird to us, we could just ask our parents and they would explain the image in a way that made sense to a little kid. I know a lot of people who are fascinated by the work, but say, 'I could never hang this in my living room...' I could hang it in my bedroom! It doesn't shock me."
Gurteen Castle is where Marilyn Manson married Dita Von Teese in 2005, in a wedding Vogue described as being filled with "high drama and high style".
Mercedes missed the wedding. "I was in LA, but I was there when they came before the wedding to visit," she says. "I'd met him a few times before that. I remember, Marilyn Manson was so big when I was a teenager, the first time I met him, it was weird... 'This person is real?' You only ever saw him on MTV. He's a very intelligent guy, very well read and educated. He definitely has his own bizarre universe, but you can have some great intellectual conversations with him. He's very calm and interesting."
The kind of precociousness that Mercedes showed in her appreciation of isolated castle life has translated into her career - she published a novel, The Potential Hazards of Hester Day, when she was just 23.
"I look at that book now," she says, "and it's a little cringeworthy. The main character in the book is 18, and it's not that the book is bad, it's just that from my viewpoint, having got a lot older in the meantime..." she pauses, I think trying to be fair, then says, "when you're younger, you go all out with your style, and you say more than you need to say. With writing, I think you can get a lot better with age. More refined."
Soon though, she switched to art. "Writing books is not extremely lucrative," she says frankly. "It takes ages to finish it, then get it to the point where it's published, so I was always doing art to make a living off it. I was having shows, getting collectors. I like the idea that you can switch back and forth between two totally different things."
Was her father much of an influence on her work? "When I was a kid, I was drawing a lot and all through my teenage years, without any major comments or interference from him. I started having art shows and people were interested in the drawings and I became a little more serious about them, but I never went to art school and I never went to my dad to learn a technique.
"I grew up with his work, so that influenced me. But it was a general inspiration, rather than anything specific. I developed my own way of doing things, but I like getting his feedback or reaction. Visually speaking, I don't know anyone whose decisions I would trust so much."
At one of her early shows, a group exhibition in London, Damien Hirst bought everything she put up. "That was one of those little surreal moments in life. I've never met him. I didn't go to the opening, I was busy, doing a different show somewhere else. I wasn't even in the same country. It was bizarre, it was something that belonged to a sci-fi. My mum saw him in Mexico after that, and he wrote a little note for me, and that was great."
For all that writing and drawing have always been a huge part of her life, music, Mercedes says, is primary. "There's something about music that saves my life 20 times a day. I still get into it like I would if I was 15." What does she mean 'saves' her life? "I don't think anyone's life is easy. The freedom you get from listening to the right song at the right point in time will elevate you out of all the stuff that's going on."
It was music that initially attracted her to husband Chris. "He's a really good musician - I would go and see his band and we'd talk about music," she says. The couple got married in Tipperary some years ago, or rather, tried to. "I got married in a ceremony that wasn't legal, because we forgot to register," says Mercedes.
"We had friends, the ceremony, everything, but it was just a party. Then we did the legal thing in LA years later, a drive-through place where you can get married in half an hour."
Right now, Mercedes is working on a second novel - "I thought I had finished it, and I had an editor go over it, and after the notes I thought 'Nope...' so now I'm almost at the end again. I'm pretty excited to be done with it" - so her working day is divided. "I try to give myself until the middle of the day to write, then go to my studio and work on the other side of things. Then there's the business side too."
Are religion and spirituality important to her? "Yes, very. That's where art comes in for me. I feel that's what actually keeps people alive and enables them to deal with life.
"To me, honestly, so much of spirituality is tied in with art and being able to exist and deal with all the hardship that's just a normal part of life. There are a lot of sad things that go on, things that are wrong with the world on a grand scale or on a small scale, and that's why I think art shouldn't be pushed aside as a cute thing or as a fun thing or as a decorative thing. I honestly think it's what makes it possible to be alive."